Combination convection/steamer ovens, which are hybrids of conventional convection ovens and steamers, have been favorably received in the food service and cooking industries because of the shortened cooking times required with such ovens and the quality of food produced. Combination ovens cook some food at speeds rivaling those of microwave ovens yet without the quality problems often associated with microwaved food. Due to the ability of combination ovens to retain moisture in food much better than conventional ovens or microwaves, overcooking and the associated drying and shrinkage of food is much easier to avoid. In addition, by better retaining moisture in food, combination ovens produce healthier food that has not been cooked free of vitamins and other nutrients that are often lost during conventional cooking.
A typical combination convection/steamer oven includes a cooking cavity and a convection air heat source similar to a conventional oven. However, a combination oven also includes a steam generating system, which may include an auxiliary heating element to generate steam or may use the convection air heat source to generate steam. One key requirement for effective steam cooking is maximum steam saturation in the cooking cavity of the combination oven. Steam is a much better conductor of heat than is hot, dry convection air, thereby giving rise to the aforementioned fast cooking times. One pound of steam has an energy content of approximately 970 B.T.U.'s. However, even a slight amount of air present in a steam-filled cooking cavity markedly reduces the cooking ability of the steam, because the air acts as a layer of insulation around the food, preventing efficient heat transfer from the steam to the food. In cooking with steam, it is therefore desirable to maintain the highest possible steam saturation for the fastest possible cooking of food.
Another consideration involved in steam cooking with a combination convection/steamer oven is quenching. Quenching can occur when too much water is introduced into the system or when large loads are put into the oven, thereby overloading and cooling the heating elements that create the steam. A fine line exists between introducing too little water to the system, thereby preventing steam saturation and causing the associated inefficient heat conduction to the food, and providing too much water to the system, thereby causing quenching and the resultant cooling associated therewith. Therefore, to maximize the performance of a combination convection/steamer oven, the amount of water introduced into the system should match the maximum steam generating capacity of the system.
A typical gas-fired combination convection/steamer oven is disclosed in U.S. Pat. No. 5,014,679 to Childs et al. (hereinafter Childs), which includes a heating chamber, gas burners, a burner-heat exchanger that isolates combustion gases from the heating chamber, a water supply, and an atomizer for producing an atomized water spray that circulates over the surface of the heat exchanger to generate steam for cooking food in the heating chamber. The Childs oven regulates the steam content and temperature in the heating chamber by measuring the temperature of steam exiting a drain channel below the heating chamber. When the temperature sensor detects a drop in the drain temperature caused by a diminishing steam content in the oven, the sensor signals a controller to generate additional steam by turning on the water supply. Thus, the Childs oven uses a thermostat-type of steam generating system that supplies water to the system when the drain temperature drops below the preset cooking temperature and cuts off the water supply when the drain temperature rises above the selected cooking temperature.
A major disadvantage of the Childs oven is that it does not adjust the rate of water flowing into the steam generating system at startup or after introducing a cooking load into the oven, but only turns the water on or off depending on the temperature in the drain. Therefore, the quantity of water in the Childs oven fluctuates, which alternately leads to a lack of steam saturation and quenching, both of which are undesirable. Another disadvantage of the Childs oven is that the controller of Childs compares the drain temperature to the preset cooking temperature when determining whether to turn the water on or off. This often leads to inaccuracies when the actual temperature of the heating chamber is different from the desired preset temperature due to aging of the heating elements, scale build-up on the heat exchanger, heat loss through the walls of the heating chamber, and other unavoidable factors that hinder accurate operation.
A similar combination convection/steamer oven design is shown in the patent to Miller, U.S. Pat. No. 4,700,685, which uses electric heating elements instead of gas burners. However, like Childs, the Miller oven does not adjust the flow rate of water into the system; in fact the Miller oven continuously supplies water to the steam generating system and does not turn the water off during operation. U.S. Pat. No. 5,368,008 to Oslin discloses a steamer apparatus that adjusts the rate of steam entering the cooking chamber by varying power to the steam generating system. The Oslin steamer cycles between a cooking mode using high power and a quiescent mode using lower power. The Oslin device does not vary the rate of water flow.
Accordingly, up until now, there has not been a combination convection/steamer oven that adjusts the water flow rate to maximize the performance of the oven by continuously maintaining optimum steam saturation levels regardless of inherent inaccuracies and variations present in the oven.